The Light Eaters: How the Unseen World of Plant Intelligence Offers a New Understanding of Life on Earth

by Zoë Schlanger

Plants are the very definition of creative becoming: they are in constant motion, albeit slow motion, probing the air and soil in a relentless quest for a livable future.

Von Humboldt wondered aloud why being in the outdoors evoked something existential and true. “Nature everywhere speaks to man in a voice that is familiar to his soul,” he wrote; “Everything is interaction and reciprocal,” and therefore nature “gives the impression of the whole.”

THEY SAY TRYING to understand any culture is like looking at an iceberg: there are vast depths you cannot see.

To many people I spoke to, “intelligence” felt dangerous, yes, but only because most people make a mental leap directly to human intelligence. Measuring plants against human cognition made no sense; it just rendered plants as lesser humans, lesser animals. Anthropomorphizing was dangerous because it diminished these green bodies, leaving no room for the recognition that plants deploy several senses—or could one say, intelligences?—that far exceed anything humans can do in a similar category. Our versions of those senses, if we even have them, are paltry in comparison. It was hard for these researchers to talk about plant intelligence; they worried that it was a trap, leading to conclusions that did not represent the real wonder of what they’d learned.

The roiling plasma surface of the sun flings out a fistful of light. The particles—billions of photons—hurtle 93 million miles through black space to rain down like bread and honey on the outstretched flesh of the most abundant living mass on earth. Plants eat light. Photosynthesis, so basic to plants, is the prerequisite for most every other life form on earth. Through photosynthesis, plants suffuse the air with the oxygen we breathe. How did we get here? A billion and a half years ago, an alga-like cell swallowed a cyanobacteria. That alga-like cell was the early organism from which both animals and fungi would later evolve, and the cyanobacteria is an ancestor of the unthinkably diverse bacteria that flood our world today. But together they were the start of a new branch of life entirely.

Their supremacy is absolute. If weighed, plants would amount to 80 percent of Earth’s living matter.

It’s not a stretch to say they birthed the habitable world. As the Italian philosopher Emanuele Coccia puts it, they constructed our cosmos; “The world is, above all, everything the plants could make of it.” Through the same process, plants have made every iota of sugar we have ever consumed. A leaf is the only thing in our known world that can manufacture sugar out of materials—light and air—that have never been alive.

As photons from the sun fall upon a plant’s outstretched green parts, chloroplasts in the leaf cell convert the particle of light into chemical energy. This solar power gets stored inside specialized energy-storing molecules, the rechargeable battery packs of the plant world. At the same time, the leaf siphons carbon dioxide out of the air through minuscule pore-like openings on the underside of the leaf, called stomata. Under a microscope, stomata look like small parted mouths, fish lips that open and close. They are breathing, after all, in their way. The stomata suck in carbon dioxide, and the carbon dioxide now encounters both the stored solar energy in the chloroplast and the water that is coursing, always, through the leaf’s veins. Through that encounter with the pure energy of light, the water and carbon dioxide molecules are ripped apart. Half of the oxygen molecules from both parties float away from this meeting, passing back out into the world through the parted lips of the stomata—becoming the air we breathe. The carbon, hydrogen, and oxygen that remains is spun into strands of sugary glucose. To be precise, it takes six molecules of carbon dioxide and six molecules of water, torn apart by power from the sun, to form six molecules of oxygen and—the true aim of this whole process—one precious molecule of glucose. The plant uses the glucose to build new leaves, which will be used to make more glucose. It also shuttles the glucose down through its body, passing it ...

Think about it: every animal organ was built with sugar from plants. The meat of our bones and indeed the bones themselves carry the signature of their molecules. Our bodies are fabricated with the threads of material plants first spun. Likewise, every thought that has ever passed through your brain was made possible by plants.

All the glucose in the world, whether it arrives in your body packaged inside a banana or a slice of wheat bread, was manufactured out of thin air by a plant in the moment after photons from the sun fell upon it. In this way we are, at every moment, brought into conversation with plants, and they with us. Our thoughts, and the products of them—the fabric of our cultures, the direction of our invention—have behind them trillions of plant bodies, each alchemizing the world into existence.

Every native plant on Kaua‘i, Hawai‘i’s fourth-largest island and Perlman’s home base, is a mind-blowing stroke of luck and chance. Each species there arrived on the island as a single seed floating at sea or flying in a bird’s belly from thousands of miles away—more than two thousand miles of open ocean sit between Kaua‘i and the nearest continent. Botanists believe one or two seeds made it every thousand years.

When plants are allowed to evolve without fear, they get scrupulously and flamboyantly specific. Take the Hibiscadelphus genus, for example. Found only in Hawai‘i, these plants have long tubular flowers, custom-made to fit the hooked beak of the honeycreeper, the precise bird that pollinates them.

Any sense that immobility implies passivity is quickly banished by a look at plants’ vast capacity to make chemical weapons. Plants are themselves synthetic chemists, surpassing the best human technology in terms of the subtle complexity of the chemicals they can synthesize. A leaf, sensing that it has been nibbled, can produce a plume of airborne chemicals that tell a plant’s more distant branches to activate their immune systems, manufacturing yet more repellent chemicals to deter incoming aphids and other plant-eating bugs. Several species of plants have been found to identify a caterpillar’s species by sensing the compounds in its saliva, and then synthesize the exact compounds to summon its predator. Parasitic wasps then obligingly arrive to take care of the caterpillars. But Kaua‘i’s plants have none of these defenses, or in any case, far fewer. Any that the plants’ predecessors may have had—thorns, or poison, or repellent scents—were completely dropped after they landed on the island. No large land mammals, reptiles, or other potential predators made the journey from the mainland to the remote island chain. In fact, the only land mammal native to all of Hawai‘i is a small, fuzzy bat. (The journey its ancestor must have taken from North America is almost unthinkable; it likely gusted over inside a storm.) From the plants’ evolutionary perspective, there is no reason to spend energy on defenses when there are no predators to fend off, so mint has lost their mint oil, ...

“I’ve already witnessed about twenty species go extinct in the wild,” he says. He has sat beside the last of a species and kept it company as it died.

“Humans are not the lords of this earth,” Geniusz writes. “We are the babies of this family of ours. We are the weakest because we are the most dependent.” Where Geniusz talks of linkages, of dependance and kin, most European thought is fixated on distance and detachment. Perhaps nowhere is this more clearly illustrated than in our corruption of the word vegetable, which is now a crude word for a brain-dead human being. But vegetabilis came from the medieval Latin, meaning something that is growing or flourishing. Vegetāre, the verb, meant to animate or enliven. Vegēre was the very state of being alive, being active.

The English physiologist William Harvey was the first European to accurately describe how blood circulated, thanks to dissecting live animals. (Ibn al-Nafis, an Arab physician from Damascus, beat him by a long shot, accurately describing pulmonary circulation three hundred years prior.) Claude Bernard, the renowned French physiologist, supposedly live-dissected his family dog in the 1860s. The story goes that after coming home to find what Bernard had done, his wife and daughters left him to join an early anti-vivisection society. Vivisection fell out of fashion not because science had changed its mind, but because the first animal welfare societies—led in most cases by women—sprang up to oppose it.

It took the neuroscience revolution of the 1960s for researchers to think of the “mind” as something scientists could study by watching people’s behavior, rather than by directly observing their brains. By the 1990s and 2000s, ambitious zoologists were using those techniques on dolphins, parrots, and dogs. They found that elephants could recognize themselves in the mirror, that crows make tools, and that cats exhibit the same attachment styles as human toddlers. Today, just four decades since Griffin’s plea to his field, it isn’t heresy to talk about animal cognition, to study the behaviors of individual animals, and to ascribe to them personalities. In fact, it’s approaching the mainstream. In 2012, a group of scientists gathered at the University of Cambridge to formally confer consciousness on all mammals, birds, and “many other creatures, including octopuses.” Nonhuman animals had all the physical markers of conscious states, and clearly acted with a sense of intention. “Consequently, the weight of evidence indicates that humans are not unique in possessing the neurological substrates that generate consciousness,” they declared.

New research suggests bees may have a form of subjectivity, a marker used by some to denote consciousness. Where further down to look, past insects? How about plants? Right now, one camp of botanists is arguing that it’s absolutely time to expand our notions of consciousness and intelligence to include plants, while another argues that’s an illogical road to go down. Many more botanists are sitting in the middle, quietly doing remarkable work, waiting to see how the larger debate falls out.

Plants were clearly more than decorative. They seemed almost like companions.

Then, in the 1860s, Charles Darwin became captivated by plants. By then he was already a famous man. It had been a number of years since he’d published On the Origin of Species, and island voyages, exotic animals, and volcanic geology seemed to better suit a younger him. As an older man, he shifted his focus to nearer things, right at his feet: almost all of his books after Origin were about plants.

Of course plants don’t have neurons or brains. But research was suggesting they might have analogous structures, or at least some physiology that could do similar things, and a cognitive capacity that deserved to be taken seriously. Plants produce electrical impulses, and seem to have nodes at the tips of their roots that serve as local command centers. Glutamate and glycine, two of the most common neurotransmitters in animal brains, are present in plants also, and seem to be crucial to how they pass information through their stems and leaves. They have been found to form, store, and access memories, sense incredibly subtle changes in their environment, and send highly sophisticated chemicals aloft on the air in response. They send signals to different body parts to coordinate defenses. Plant neurobiology “aims to study plants in their full sensory and communicative complexity,” they wrote.

Communication implies a recognition of self and what lies beyond it—the existence of other selves. Communication is the forming of threads between individuals. It’s a way to make one life useful to other lives, to make oneself important to other selves. It turns individuals into a community. If it is true that a whole forest or field is in communication, it changes the nature of that forest or field. It changes the notion of what a plant is. What is a plant without a means to communicate? A husk. And without conversation, a forest is not a forest.

For twelve full pages Rhoades dutifully copied out pupal weights of caterpillars and leaf losses from trees. He’d been watching the university experimental forest get decimated by an invasion of tent-forming caterpillars for several years, he explained. But suddenly something had changed; the caterpillars began to die. Why, he asked, did the voracious caterpillars suddenly stop munching, leaving tree leaves intact? Why did they seem to abruptly die out? The answer, Rhoades discovered, was improbable, remarkable, and dangerous: the trees were communicating with each other. Trees the caterpillars hadn’t yet reached were ready; they’d turned their leaves into weapons. The caterpillars that ate them got sick and died. Communication between trees via their roots had been established somewhat earlier than Rhoades’s finding, but this was different. The trees were too far apart to be passing information through their roots. The message—that the caterpillars were coming—was getting through regardless. His excitement at what that implied was more than he could dampen. When all room for dry description was exploited, and there was nothing left to do but say it, Rhoades could not help but let the true nut of the paper out as a chirp, using that most exposing punctuation: “This suggests that the results may be due to airborne pheromonal substances!” The trees were signaling to each other, he said, across long distances, through the air.

To this day, in order to coalesce into a body, each cell in an organism must know who it is and what it does. Cells understand themselves by way of other cells; in a chain of three cells, for example, the third cell knows it is third—and thus endowed with a special task reserved for third cells—because of its awareness of the presence of cell one and cell two. That is the nature of a self-organizing system, a cohesive body. But how that cell knows it is third remains a mystery. We know that information must be passed to it by its collaborator cells. This communication, whatever it is, begins with the very first cell division, when one cell becomes two and then four, which is the strategy every multi-celled organism uses to grow. The medium of that information—electrical? Chemical? Some other form?—is unknown.

As every ecologist knows, nothing changes in an ecosystem without a reason. Something drove that shift.

Everyone already knew that ripening fruit produced airborne ethylene, for example, which prompts nearby fruit to ripen too. The commercial fruit industry used it to ripen warehouses full of unripe bananas just in time for sale, making the global trade of an otherwise fast-rotting fruit possible.

He’d come up studying cicadas, which lay their eggs in trees. When the larvae hatch, they drop to the ground, burrow into the tree’s roots, and stay there for seventeen years, sucking its sap. To the tree, it’s a huge nuisance to have all that nutrition leak out of its lower parts before reaching its upper ones. As a young scientist, Karban read a paper by pioneering cicada researcher JoAnn White, who discovered that some trees were able to locate the place on their branch where the cicada eggs sat and grow a callus around them, suffocating them to death before they could hatch. Karban, like Rhoades, thought that plants couldn’t possibly be passive.

Baldwin and Schultz placed pairs of sugar maple seedlings inside the sterility of a growth chamber. The seedlings shared the same air but didn’t touch. Then the researchers ripped the leaves of one and measured the response in the other. Within thirty-six hours, the untouched maple seedling loaded up its leaves with tannin. In other words, despite not experiencing damage itself, the untouched maple went to work making itself extremely unpalatable. Baldwin and Schultz noted that they were the second in line to notice this phenomena, acknowledging Rhoades in their paper. They even went so far as to use the word communication in their work (Rhoades never used the c-word, choosing to dance around it). The mainstream press understandably seized on the wording, printing headlines about “talking trees” in national newspapers. They were generally chastised by peers for using such human language for plants, but in contrast to Rhoades, to say their careers recovered would be an understatement. Today Baldwin is one of the most successful and prolific individuals working on plant behavior. He has a large team of graduate students and postdocs working out how tobacco plants, in effect, communicate, defend themselves, and choose which other tobacco plants to have sex with. Jack Schultz spent decades as a major contributor to the field of communication between plants and insects, and was known to say that the scent of cut grass is the chemical equivalent of a plant’s scream. Both say the...

The world is a prism, not a window. Wherever we look, we find new refractions.

The way he sees it, nature had basically decided, “I’m going to have to reduce the population of these animals,” he says. “And then it did it.”

Sure enough, the damaged trees were releasing great plumes of ethylene, certainly enough to waft over to a nearby tree. Surrounding trees were being alerted and changed their behavior accordingly, he decided. It was a coordinated poisoning.

At the time of my visit, Finnish evolutionary ecologist Aino Kalske, Japanese chemical ecologist Kaori Shiojiri, and Cornell chemical ecologist André Kessler had recently found that goldenrods that live in peaceful areas without much threat from predators will issue chemical alarm calls that are incredibly specific—decipherable only to their close kin—on the rare occasion they are attacked. But goldenrods in more hostile territory signal to their neighbors using chemical phrases easily understood by all the goldenrod in the area, not just their biological kin. Instead of using coded whisper networks, these goldenrod broadcast the threat over loudspeaker, so to speak. It is the first time research has confirmed that these sort of chemical communications are beneficial not only to the plant receiving them but also to the sender.* When times are truly tough, you don’t want to be left standing in a field alone when it’s over, if you’re a plant. There’ll be no one to mate with, no one to help bring in pollinators. It’s the closest scientists have come to showing intentionality in plant communication: these are signals meant to be heard. And as we know, by some measures, intention is an indicator of intelligent behavior.

Sagebrush also use “private” means of communication to warn only their family groups about insect attacks when the threat from bugs was generally low. Basically, they are using backchannels—the chemical compounds they use are complex and specific to them and their closest allies. But when the whole community is being heavily attacked, sagebrush will switch to “public” channels, emitting more universally understandable alarm calls. This tracks perfectly with something that has been known about songbirds for a long time. In peaceful places, where relatively few dangerous predators lurk, birds use extremely specific song phrases to warn only their family group that something is wrong. But when the birds are facing widespread danger, they switch calls, making alarm sounds that everyone in the area can understand, even members of other bird species. This makes sense, again, in terms of community survival; when the whole neighborhood is threatened, it’s best to save as many of your kind as possible, regardless of if they’re family or not.

That means plants could be said to have dialects, and are alert to their contexts enough to know when to deploy them. More than that, they have a clear sense of who is who; who is family, and who is not. They are in touch with their surroundings, and with the fluctuating status of their enemies. Their communication is not just rudimentary but complex and layered, alive with multiple meanings.

Personality research entered the world of zoology relatively recently; in the last 20 years, animal science has begun to take seriously the idea that individual animals have personalities—consistent, unique ways of responding to the world—and that they are worthy of study.

Sometimes a plant will signal distress, and its neighbors won’t produce defensive compounds, or sometimes they just produce less. Karban believes this may be because individual plants may have different tolerances for risk—one metric of personality. Some, he says, might exhibit a personality akin to, say, natural-born scaredy-cats; they will signal wildly at the slightest disturbance. In that case, other plants in the same family will treat their scared kin like the boy who cried wolf and ignore them. They won’t produce compounds of their own.

Chipmunks have distinct distress calls; one for when they detect an aerial predator, like a hawk, and another for a land predator. Some chipmunks, she said, would squeal all the time. “Some guys would be eating seeds, and a leaf falls on the ground. They panic and they make a call,” she said, screaming their little heads off about an imagined bird of prey. Those were the shy ones. “Some guys just keep foraging.” When she controlled for sex, social status, and age, there were still distinct differences in chipmunks’ personalities that remained stable over time.

What the other chipmunks choose to do with that information seems to depend on how reliable the squaller was. “The main idea is that if you have some guys who cry wolf all the time, they shouldn’t be trusted.” She recorded calls from a range of chipmunks who fell on different places along what she and her colleagues called the “shyness-boldness continuum,” and played them back to other chipmunks. The listeners perked up and paid attention when they heard a distress signal coming from a bold chipmunk, and seemed not to care as much about one from the oft-panicked.

Much like in humans, where the mind is studied by inference—what a person does—rather than neurological mechanisms, Karban is looking for patterns in behavior. “I’m a big fan of using what decades of psychology has learned, their methods, and asking if they apply to plants,” he says. “In some cases that’s no, and that’s fine.” But he’s found one method from the field of psychology research that truly seems to fit. The method helps researchers analyze behavior by separating it into two processes. The first is judgment, or the perception of raw information; the second is decision-making, or how one weighs the costs and benefits of different actions and selects the best one. This applies perfectly well to plants, he says.

“People ask me, do plants feel pain?” he says in response. But the question misses the point. “Plants know they’re being eaten. They probably experience it very differently than we do.

New layers of soil complexity have more recently come into focus, involving interspecies relationships between untold numbers of microbes and fungi. Plant personality could be yet another level of that complexity.

Agricultural researchers have warned of the dangers of monocultures—planting a single genetic variety of crop over large swaths of land—ever since the mid-nineteenth century, when a microbe caused a disease known as potato blight which proved particularly deadly to the Irish Lumper, a staple food crop in Ireland at the time. The devastation of the potato harvest caused mass hunger and around one million deaths. Still, given the economics of modern agriculture, which values yield above all, many of the world’s food staples continue to be grown in vast, undifferentiated fields. The crops tend to be bred for productivity above anything else, often at the cost of other traits, like the ability to defend themselves. As such, huge quantities of pesticides and fertilizer are often needed to sustain them.

Plenty of research has already shown the benefits of biodiversity to the resilience of a farm field or an ecosystem.

A field of multiplicity may flourish precisely in relationship to the many approaches to life it contains. As these initial findings illustrate, neither the meek nor the bold can perpetuate a species on their own.

He attached electric probes to various vegetables, and claimed to record a “death spasm” in the form of a spike in electrical activity. He hooked a cabbage to a voltmeter in front of the playwright George Bernard Shaw, who was reportedly horrified to witness the electrical “convulsion” of the cabbage as it was dropped in boiling water. Shaw, it must be said, was a vegetarian. Bose also observed how mimosas produced an electric impulse just before their leaflets closed. English scientist John Burdon-Sanderson had first recorded “electrical excitations” in another sensitive plant, the Venus flytrap, in 1876.

Plant and animal bodies may be operating on similar basic principles, at least electrically speaking.

Now there was proof of ion channels. This significant finding should have marked a turning point toward a lustrous career in what would have amounted to a brand-new field of research. But at that point, plant behavior was emaciated of funding yet again. In 1995, then-president Bill Clinton got word that the U. S. Department of Agriculture was funding studies on “stress in plants” with taxpayer money. He even made a jibe about it in that year’s State of the Union Address, implying that he thought the study was about plants needing psychotherapy, and promised to cut such wasteful spending. This general attitude morphed into added skepticism for researchers trying to push the envelope on plant physiology. Funding became harder to get.

The one exception to this sense of mystery is the Venus flytrap, the subject of some of the earliest plant electricity experiments. The plant is famous for the almost animal impression it gives when it closes its trap, which had just been hanging open toothily, quite like a mouth, a moment before (in reality, the trap is a leaf with a hinge). It eats what we can identify as “real food”—insects, like flies—in addition to having the nearly magical plant habit of photosynthesis. It is a delight to watch one of these leaf-maws spring closed, confirming its carnivorous prowess—how has a plant, in a great reversal of the usual fortunes, fatally outwitted an animal? Of course, this happens all the time in slower ways—one need only remember the starving caterpillars, poisoned slowly by leaves in revolt—but we mammals are temporally biased in one direction. We love a quick kill. The inside of each trap bristles with a few flexible spike-like hairs. Insects, lured by a sugary aroma, brush against the hairs while looking for nectar. In 2016 researchers discovered that these hairs are mechanosensory switches that elicit action potentials, and that the flytrap can actually count how many action potentials have been triggered; an electrical burst registers on a voltmeter when a hair is touched, and the maw snaps shut. Just to be sure, the researchers zapped the flytraps with doses of electricity, not touching their hairs at all. The flytraps closed just the same. This is the clearest ex...

The class covers all the basics, but with a distinctly plant-forward flair. When he gets to the Great Oxygenation Event—the long period in which the earth’s atmosphere transitioned away from being a suffocating cage of carbon dioxide to an oxygen-dominated haven—he makes sure one crucial detail sinks in: plants did that. They made the terrestrial world a habitable place for other forms of life to arise, and eventually to be able to breathe. Without them, animal life as we know it would not even have had the faintest shot at clambering onto the evolutionary treadmill. Our cells would never have formed. “Things like mitochondria wouldn’t work in the ancestral environment.”

That is quite literally the essence of the entire question of plant intelligence: How does something without a brain coordinate a response to any stimuli at all? How does information about the world get integrated, triaged by importance, and translated into action that benefits the plant? How can the plant sense its world at all, without a centralized place to parse all that information?